1. Field of the Invention
The present invention relates to an image reading device and an image forming apparatus.
2. Description of the Related Art
In an image reading device such as a scanner, a photoelectric conversion unit reads an image on an original as an analog image signal and a signal processing unit performs analog signal processing including amplification to convert the analog signal into a digital signal. Thereafter, various types of image processing are performed on the digital signal.
FIG. 9 is a schematic diagram of a typical image reading device. The image reading device includes an exposure glass 2 on which an original 11 is placed. The exposure glass 2 is arranged on an upper surface of a scanner chassis 1. The image reading device further includes an optical scanning system that is arranged below the exposure glass 2 and inside the scanner chassis 1. The optical scanning system includes a first carriage 4 that includes a light source (lamp) 3 and a first reflecting mirror 4a, a second carriage 5 that includes a second reflecting mirror 5a and a third reflecting mirror 5b, and a lens unit 6.
The image reading device further includes an image sensor 8 that is mounted on a sensor board 7, and a signal processing unit 10 that processes an image signal received via a signal cable 9. A shading plate 12 for correcting various types of errors is arranged in front of the exposure glass 2.
When reading the original 11, a stepping motor (not shown) drives the first carriage 4 and the second carriage 5 to move them in a direction indicated by an arrow A shown in FIG. 1, i.e., in a sub scanning direction. The stepping motor moves the second carriage 5 at a speed half of that of the first carriage 4. While the first and second carriages 4 and 5 are moving, a lower surface (image surface) of the original 11 is exposed to light from the light source 3.
The light reflected from the lower surface of the original 11 is sequentially reflected and deflected by the first reflecting mirror 4a, the second reflecting mirror 5a, and the third reflecting mirror 5b and is guided to the lens unit 6. The lens unit 6 converges the light on the surface of the image sensor 8. As a result, a reduced image of an image read from the original 11 is formed in the image sensor 8. The image sensor 8 converts the light (image) into an electric signal corresponding to density of the image (intensity of the light) and outputs the electric signal as an analog image signal (voltage signal). The analog image signal is output from the sensor board 7 and input to the signal processing unit 10 via the signal cable 9.
A xenon lamp or a halogen lamp has been generally used as a light source of such an image reading device. However, halogen lamps and xenon lamps are expensive and big. Therefore, now days, cold cathode fluorescent lamps (CCFL) are preferred over halogen lamps and xenon lamps from a viewpoint of cost, size, and service life.
However, a warm-up period of CCFLs is longer than that of xenon lamps. The warm-up period is a time period from a time point at which a CCFL is turned on to a time point at which the output of the CCFL is stabilized, i.e., the light amount emitted from the CCFL is stabilized to a predetermined amount. The warm-up period is also called stabilization time or wait time. To shorten the warm-up period, one approach is to apply an initial voltage that is higher than a rated voltage to the CCFL when turning on the CCFL.
For example, Japanese Patent Application Laid-open No. 2005-45309 discloses a technology for shortening the stabilization time. When a CCFL is turned on, an initial voltage higher than a rated voltage is applied to the CCFL. At every predetermined time after the CCFL is turned on, an image sensor for reading an image detects an amount of light emitted by the CCFL. When the variation in the amount of light during a predetermined period is lower than a predetermined value, an analog image signal output from the image sensor is adjusted to a gain value and a white balance is adjusted for an analog-to-digital (A/D) conversion. Thereafter, application of the rated voltage to the CCFL is started, so that the CCFL is ready for reading the image.
However, if the initial voltage higher than the rated voltage, the amount of light emitted from the CCFL increases immediately after the CCFL is turned on. Thus, the image sensor outputs an analog image signal (voltage signal) having a voltage higher than that of an analog image signal output when the output of the CCFL is stabilized, i.e., it stably emits a predetermined amount of light. Therefore, the analog image signal exceeds a tolerance of the input voltage of an analog signal processing unit (analog front end (AFE)) that performs processing, such as amplification, on the analog image signal and converting the analog image signal into a digital signal to be output to a signal processing unit. This may damage the image reading device.